Rhesus monkey alloantisera raised by planned immunization were evaluated for lymphocytotoxic antibodies by (1) titration against peripheral blood lymphocytes of selected monkeys; (2) typing of random unrelated monkeys and of monkey families by standard microcytotoxicity methods; (3) absorption analysis. Such methods have resulted during the past year in an increase of recognizable RhLA serologically defined antigens from 21 to 24 including 12 in the A series and 12 in the B series. DR typing with primed lymphocytes has been compared with DR typing by homozygous typing cells. Both appear to detect the product(s) of a locus or loci linked to RhLA in family studies, but disparities between the two methods in random unrelated monkeys suggest there may be at least two loci. Seven antigens expressed predominantly on B lymphocytes have been studied. Four are represented in the monkey families and these behave genetically as alleles of a single RhLA-linked locus.